Blender 2.8 Beta: No compatible GPUs found on iMac (2018)
There is no OpenCL support for Blender 2.80 BETA on macOS and it likely won't come back in 2.80 RC or Final. 


  Hi,
  
  There was a growing payload of bugs in Cycles related on OpenCL on macOS
  platform, and those issues were caused by a compiler bug, which we have no
  control over.
  
  Surely, it is sometimes possible to work compiler bugs around from a
  source, but we are facing some of the issues which are not solvable in this
  way. Also, such solutions are usually short-living,. since adding more
  features are often kicking compiler to provide buggy binary again.
  
  In this case compiler will not get fixed since Apple decided to discontinue
  OpenCL on its platform.
  
  So the decision was made to drop support of OpenCL, keep official features
  of Blender stable and predictable, and focus on things we have control over.
  
  P.S. Older Blender releases are always available. Surely, this sounds like
  using an ancient software without neat features. But we can't push Cycles
  OpenCL on macOS measurably beyond that anyway.
  
  -- 
  With best regards, Sergey Sharybin


Apple is known for doing everything to make software development - especially 3D stuff - a living hell. They are doing their own stuff called Metal, instead of using a complete OpenGL/OpenCL compatible instruction set. With every new release functions are getting removed or are simply broken. 

If you want to use Blender with an OpenCL render device consistently in the future, you might need to switch over to Linux or Windows.